Application Of Laser Induced Breakdown Spectroscopy In The Detection Of Soil And Plant Information

Soil is the most important component of ecosystem,and it plays an important role in agricultural production and human activities.In recent years,the rapid development of industry and agriculture and the frequent use of agrochemicals have caused serious soil heavy metal pollution,which has led to the decline of crop yield and quality,resulting in huge economic losses.The establishment of a sound soil heavy metal pollution monitoring and prevention system is helpful to solve the pollution problem,improve environmental quality,ensure food safety and promote social progress.The development of a rapid and convenient technique for the detection of soil heavy metals is one of the key links.The traditional detection method of heavy metals is chemical analysis,which consumes a lot of resources,and there is a big gap between it and rapid real-time detection.Laser induced breakdown spectroscopy(LIBS)is a new spectroscopic technique for elemental analysis.It has low requirement on sample pretreatment,and can realize simultaneous detection of multiple elements.Besides,It can meet the requirement of real-time and online fast analysis of elements,and has been widely used in various fields.LIBS combines with signal enhancement technology can improve the detection performance,reduce the detection limit of elements,and ensure the detection accuracy of heavy metal pollution in the actual environment.In this study,detection methods of soil plant information based on LIBS technology was studied,with emphasis on the detection of soil cadmium and lead pollution.The influence of laser wavelength on the characteristic spectral lines of elements were explored,and the optimal detection wavelength was determined.The environmental regulation assisted enhancement LIBS technology,the conductive material assisted enhancement LIBS technology and double-pulse LIBS technology were applied.The quantitative detection models of soil cadmium and lead content were established.This study provided theoretical basis and technical support for the establishment of monitoring,prevention and control system for soil heavy metal pollution.The main conclusions were achieved as follows:(1)Methods for the detection of soil and plant information based on LIBS technology were studied,and rapid analysis models were established.The univariate and multivariate analysis models of soil cadmium and lead content were established by LIBS combined with chemometrics method.PLSR models obtained the best prediction results.Correlation coefficient of calibration set(Rc)and prediction set(Rp)of cadmium and lead quantitative prediction were more than 0.93.Discriminant models of the transgenic maize were established.ELM model performed best,and the recognition accuracies of modeling and prediction set reached 100%.Discriminant models of the categories of the biomass pellets and prediction models of combustion characteristic indexes were established.ELM model obtained the best discrimination result,and the recognition accuracies of modeling and prediction set were 96.88%and 96.88%respectively.LS-SVM model obtained the best prediction result,and Rc and Rp were above 0.97.(2)In order to solve the problem of high detection limit of LIBS,the optimization of laser wavelengths was proposed to improve the detection accuracy of soil cadmium and lead content.Detection models of soil cadmium and lead content under different laser wavelengths were established,and the optimal system parameter for LIBS quantitative detection of soil cadmium and lead were determined.The experimental parameters(delay time,laser energy)under different laser wavelengths were optimized.The influence of laser wavelength on the LIBS signal of soil cadmium and lead was compared,and the principle of the influence of laser wavelength on the LIBS signal was analyzed.The univariate and multivariate models for the quantitative analysis of soil cadmium and lead content under different laser wavelengths were established,and the optimal laser wavelength and models for LIBS quantitative detection of soil cadmium and lead content were determined.For the quantitative detection of soil cadmium and lead content,the sensitivity,stability and performance of quantification models under the wavelength of 532 nm were relatively good.The univariate model based on Cd II 214.44 nm and Pb I 405.78 nm reached the best prediction performance,respectively,with Rp of 0.971 and 0.986,and limit of detection(LOD)of 15.075 mg/kg and 28.862 mg/kg.(3)In order to solve the problem that external environmental signals affect the stability of LIBS detection,A method for enhancing LIBS signals of soil cadmium and lead by inert gas was proposed.Ddetection models of soil cadmium and lead content were established,which provided a new method for establishing a stable and reliable soil heavy metal monitoring system.Ambient gas affects the plasma parameters generated by the interaction between the laser and the sample,which further affects the LIBS signal.In this study,the effects of different ratios of helium(He),argon(Ar)and air on the spectral intensity and parameters of the characteristic lines of soil cadmium and lead were investigated.The optimal gas environment was determined,and the principle of signal enhancement by ambient gas was analyzed.The univariate and multivariate detection models of soil cadmium and lead content were established,and the effects of gas environment on the quantitative detection of soil cadmium and lead content were compared.For the quantitative detection of cadmium,the spectral lines had good sensitivity and stability when the ratio of Ar and He was both50%.Under this condition,the quantitative prediction performance of soil cadmium content was obviously better than that in air,and the least square support vector machine(LS-SVM)model achieved the best prediction result,with Rp of 0.962.Univariate model based on Cd I 228.80 nm had the best detection performance,with LOD of 25.023mg/kg.For quantitative detection of soil lead content,the spectral line had the best sensitivity,stability and intensity when the ratio of Ar and He was 90%and 10%respectively.Under this condition,the quantitative prediction performance of soil lead content was obviously better than that in air.The univariate model based on Pb I 405.78 nm obtained the best result,with Rp of 0.988,and LOD of 32.97 mg/kg.(4)In order to solve the problem of obvious matrix effect in LIBS detection of soil,a method based on conductive materials to enhance LIBS signals of soil cadmium and lead was proposed.Detection models of soil cadmium and lead content were established,which provided theoretical reference and method support for the expansion of LIBS application.Conductive materials can improve the physical properties of the samples,reduce the matrix effect,and further enhance LIBS signal.In this study,the effects of conductive materials of sodium chloride(Na Cl)and graphite and their contents on the intensity and parameters of characteristic spectral lines of soil cadmium and lead were investigated.The optimal Na Cl and graphite contents for quantitative detection of soil cadmium and lead were determined,and the principle of the influence of conductive materials on spectral line intensity and parameters was investigated.The univariate and multivariate detection models of soil cadmium and lead content were established,and the influence of conductive materials on the quantitative determination of soil cadmium and lead content were compared.For the quantitative detection of soil cadmium content,the sensitivity and stability of the spectral lines and the quantitative detection performance of models were better when the content of Na Cl and graphite were both 25%.The improvement effect of graphite on quantitative detection of cadmium was better than that of Na Cl.Under the optimal graphite content,LS-SVM model obtained the best prediction result,with R_C of 1.000 and R_Pof0.994.Among the two cadmium spectral lines,the univariate model based on Cd I 228.80 nm had better detection ability,with LOD of 9.923 mg/kg.For the quantitative detection of lead,the sensitivity,stability and quantitative detection performance of models were better when the content of Na Cl and graphite were 10%and20%respectively.The improvement effect of graphite on quantitative detection of lead was better than that of Na Cl.Under the optimal graphite content,Partial least squares regression(PLSR)model obtained the best prediction result,with R_C of 1.000,and R_Pof 0.993.Among the three lead spectral lines,the univariate model based on Pb I 405.78 nm had the best detection ability,with LOD of 26.142 mg/kg.(5)In order to solve the problem of low accuracy of LIBS detection of soil heavy metal,detection models of soil cadmium and lead content were established by using double pulse signal enhancement method.The proposed method realized the accurate detection of soil cadmium and lead content,and provided theoretical reference and technical support for the development of accurate and reliable portable LIBS detection system for soil heavy metals.The experimental parameters of double pulse LIBS detection were optimized,and intensity and parameters of the characteristic spectral lines of single and double pulse LIBS were compared.The principle of signal enhancement by double pulse was analyzed.Univariate and multivariate models for quantitative determination of soil cadmium and lead content were established,and the performance of quantitative analysis models of single and double pulse LIBS were compared.For the quantitative detection of soil cadmium and lead content,the quantitative detection performance of double-pulse LIBS was obviously better than that of single pulse LIBS,and the performance of univariate models was better than that of multivariate models.Among the spectral lines of cadmium,the univariate model based on Cd II214.44 nm obtained the best result,with R_P of 0.992 and the minimum LOD of 3.374 mg/kg.Among the characteristic lines of lead,the univariate model based on Pb I 405.78 nm obtained the best result,with R_P of0.995 and the minimum LOD of 22.127 mg/kg.Besides,quantitative determination of cadmium and lead of double pulse LIBS was conducted in real soil.The results were similar to the detection results of the above standard soil,and the relative performance of each spectral line were consistent.